1. Field of Invention
The present invention relates to a method for making a thin film transistor (TFT) and a structure of the same.
2. Description of Related Art
Along with the improvement of the display technology, lighter, thinner, portable, and flexible displays attract many people, and a lot of companies are involved in the research and development activities. Organic thin film transistor (OTFT) is a TFT that an organic molecule material is used to develop for electronic products. The greatest advantage of the OTFT is that when the panel is bent, the characteristics of the transistor can still be maintained to achieve a normal display quality effect. Such application may accelerate the realization of electronic products such as flexible displays.
A plastic substrate, characterized in being transparent, light, thin, impact resisting, and flexible, is suitable for the roll to roll high production rate process. Therefore, in the application of a flexible display or logic element, it is the main trend in the future to fabricate the OTFT on a plastic substrate.
FIG. 1 is a schematic view of the circuit structure of a common liquid crystal pixel. A liquid crystal pixel at least comprises a liquid crystal capacitor CCL, a storage capacitor Cst, and a TFT functioning as a switch. A gate of the TFT is connected to a scan line scan 1, a source is connected to a data line data 1, and a drain is connected to one end of the liquid crystal capacitor CCL and that of the storage capacitor Cst. Since the element structure and functions of each pixel in a pixel array are known to all, the details will not be described herein again, and only the parasitic capacitance is discussed below. In the pixel structure of FIG. 1, during the fabrication process of a plastic substrate, the water permeability and the oxygen permeability of the substrate must be improved. In addition, the soaking process of the solvent and the high temperature process during the fabrication process may cause an unstable size of the substrate, thus making an exact alignment become difficult. The above problem may raise the difficulty of the fabrication process and reduce the process yield. Particularly, the above problem may result in overlapping the source, drain, or gate during the fabrication, thus generating a parasitic effect. The parasitic effect of each region on the panel is different, so as to cause differences on picture quality. As shown in FIG. 1, usually the parasitic capacitances include the parasitic capacitance Cgd between the gate and the drain of the transistor T, the parasitic capacitance Cpd1 between the scan line and the drain, the parasitic capacitance Cpg2 between the drain and the next data line data 2, the parasitic capacitance Cpd1 between the data line data 1 and the drain, the parasitic capacitance Cpg2 between the drain and the next scan line scan 2.
During the operation, when the TFT is turned off by a gate-off voltage provided by the scan line, the voltage on the pixel electrode may suddenly drop because of the kickback voltage ΔVp. According to the following formulae, the amplitude of the kickback voltage ΔVp is relevant to the parasitic capacitances between the gate, the drain, the scan line, and the data line of the TFT.ΔVp=(ΔVgCgd+ΔVdCpd)/(Cgd+Cst+CLC+Cpd)ΔVdCpd<<ΔVgCgdΔVp=|Vgate-on−Vgate-off|×|Cgd/Ctotal|
The kickback voltage may result in the flickering of the image on the LCD. As for a common display, the smaller the kickback voltage ΔVp is, the more difficult the generation of the flickering of the frame is, and the better the display quality will be.
Generally, the voltage of a common electrode can be adjusted to reduce the kickback voltage ΔVp. However, if the alignment problem in the fabrication process is not solved to reduce the overlap between the source, drain, and gate, it remains difficult to effectively overcome the high kickback voltage caused by the parasitic capacitance effect.
Therefore, how to develop a process with preferred alignment to reduce the overlap between the source, drain, and gate and then to reduce the parasitic effect is an important issue.